NIOS-II+LAN91C111

Hi,everyone,I am now doing a work about ALTERA nios-II.The FPGA I use
is cycII-60,and the cpu is NIOS-II f,sdram ,lan91c11,dma controller
and so on,the main work I should do is to conmunicate the PCs and nios-
II with lan91c111,I chose UDP to send and receive data,also It can
work properly if I do'nt use dma to send and receive data,but the
speed is low about 18Mbps,cpu freq is 50MHZ,so I want to use dma ,but
It doesn't work ,I have a look at the soruce file ,the dma send
function have call the callback function s91_dma_tx_done(),but the PC
can't catch the udp packet,can some give me some advice ,I have no
idear.Thanks!